JPH05119053A - Hot-wire current meter - Google Patents

Abstract

PURPOSE:To lower the spatial resolution of a hot-wire current meter and, at the same time, to make the adjusting and calibrating work of the current meter easier by periodically switching the steady temperature of a thermo-sensitive sensor and detecting the velocity of a current on the basis of the difference in diffused quantity of heat. CONSTITUTION:A bridge circuit 10 is constituted of fixed resistors R1 and R2, a thermo- sensitive sensor, namely, platinum sensor 1, and a serially connected body of fixed resistors R and (r) and is maintained in a steady state by feeding back the voltage variation across its terminals by means of an amplifier 3. The contact 11a of a relay 11 is connected to one side of the circuit 10 and the fixed resistor (r) is short-circuited with the circuit 10 in accordance with a signal. The voltages across both ends of the resistor R1 and across both ends of the sensor 1 are supplied to an arithmetic processing circuit 14 after they are respectively converted into digital signals by means of A/D converters 12 and 13. The circuit 14 detects the steady state of the circuit 10 and, when the circuit 10 reaches the steady state, switches the relay 11 and, at the same, calculates the diffused quantity of heat Q and detects the velocity U of a current on the basis of the difference between diffused quantities of heat Q1 and Q2 at different steady temperatures T1 and T2 and displays the velocity U on a display 15. Q1-Q2=(a+bU<1/2>) (T1-T2), where (a) and (b) are constants.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hot wire anemometer, and more particularly to a hot wire anemometer characterized by a temperature compensation function.

[0002]

2. Description of the Related Art A conventional hot wire anemometer using a bridge circuit has resistors R1 to R1 as shown in the circuit diagram of FIG.
A fixed resistance of R3 and a platinum sensor 1 which is a temperature sensor such as a platinum wire are configured as a bridge circuit 2, and a feedback is applied by an amplifier 3 to keep the temperature of the platinum sensor 1 at a constant value. Assuming that the temperature when the platinum sensor 1 reaches a steady state is T, the amount Q of heat dissipated by the platinum sensor 1 is expressed by the following equation. Q = (a + bU ^1/2 ) (T-Ta) Q: Heat dissipation amount U: Flow velocity a, b: Constant T: Temperature of hot wire Ta: Temperature of fluid Therefore, the flow velocity U was measured from this formula.

However, in such a general constant temperature type hot wire anemometer, the amount of heat radiated changes depending on the temperature Ta of the fluid, so that there is a drawback that an error occurs in the measured flow velocity. Therefore, as shown in FIG. 5, a resistor R4 for temperature sensing is provided on one side of the bridge circuit, and the resistance value is set to be sufficiently larger than that of the platinum sensor 1. Thus, a hot wire anemometer is widely used in which the bridge equilibrium condition is automatically adjusted according to the fluid temperature and the temperature T of the platinum sensor 1 is changed to compensate for the influence of Ta.

[0004]

However, the hot-wire anemometer having such a temperature compensating element has a drawback that it is necessary to use a temperature compensating element in addition to the platinum sensor, and the adjustment of the bridge becomes complicated. .. Further, since the temperature compensating resistor R4 has a large resistance value and a large shape, the spatial resolution is deteriorated.

The present invention has been made in view of the above problems of the conventional hot wire anemometer, and enables temperature compensation using only one platinum sensor and enables reduction of spatial resolution. This is a technical issue.

[0006]

According to a first aspect of the present invention, a bridge circuit having a temperature-sensitive sensor having a resistance value corresponding to temperature connected to one end thereof, and a resistance on one side of the bridge circuit are first and first. Switch means for changing the resistance value to 2, resistance heat quantity calculation means for calculating the heat dissipation quantity of the temperature sensor when the connection of the bridge circuit reaches a steady value, and switch control means for automatically opening and closing the switch means. And a flow velocity calculating means for measuring the flow velocity based on the difference in the amount of radiated heat obtained by the amount of radiated heat obtained by the first and second resistance values connected to the bridge circuit by the switch means. It is a feature.

Further, in the invention of claim 2 of the present application, the switch control means judges the steady state when the amount of change of the signal obtained from the bridge circuit becomes a predetermined value or less and opens and closes the switch means. It is characterized by being used as a means.

[0008]

According to the present invention having such a feature, the temperature sensor is connected to one side of the bridge circuit and the resistance value of the resistor connected to the other side is changed by the switch means. Therefore, if the resistance values of the resistors are different in the state of being connected to the bridge circuit, the steady temperature of the temperature-sensitive sensor will be different in each state. Therefore, by switching the switch means by the switch control means, the steady state is periodically reached at different temperatures. Therefore, the amount of heat dissipated in each steady state is calculated, and the flow velocity of the fluid is measured based on this difference.

[0009]

1 is a block diagram showing the overall structure of a flow velocity measuring apparatus according to an embodiment of the present invention. In this figure, the same parts as those in the conventional example described above are designated by the same reference numerals, and detailed description thereof will be omitted. Also in this embodiment, the bridge circuit 10 is composed of the fixed resistors R1 and R2, the platinum sensor 1 which is a temperature sensitive sensor, and the series connection of the resistors R and r, and the voltage change at the terminal is fed back by the amplifier 3. The bridge circuit is kept in a steady state.
Here, the contact 11a of the relay 11 is connected between the fixed resistance R and the fixed resistance r on one side of the bridge circuit 10. The relay 11 is a switch means for short-circuiting the fixed resistor r according to the switch signal. The resistor R1 is a fixed resistor, and the voltage across the resistor R1 has a value corresponding to the current.
Therefore, as shown in the figure, the voltage across the resistor R1 is input to the A / D converter 12 as a current signal, and the current value is detected. The voltage across the platinum sensor 1 is applied to the A / D converter 13. The A / D converters 12 and 13 convert the voltage value input according to a control signal from the outside into a digital signal, and the output thereof is given to the arithmetic processing circuit 14. The arithmetic processing circuit 14 has an input interface and a CP.
It is composed of U and memories such as ROM and RAM. As will be described later, the arithmetic processing circuit 14 detects the steady state of the bridge circuit 10, switches the relay 11 when the steady state is reached, calculates the amount of heat dissipated, and calculates the flow rate by the difference in the amount of heat dissipated at different steady temperatures. Is detected and its output is given to the display unit 15.

Next, the operation of this embodiment will be described with reference to the flowchart and time chart. When the operation is started, first, the input values from the A / D converters 12 and 13 are cleared in step 31 of FIG. 2, and the process proceeds to step 32 where the inputs from the A / D converters 12 and 13 are fetched and stored. And either one, for example, the A / D converter 12
The difference between the input value of and the input value of one cycle before is calculated. If this difference exceeds the predetermined value ε, the process returns to step 32 and the same processing is repeated. For example, as shown in FIG. 3A, if the relay 11 is turned off and the contact 11a is opened at time t ₁ at the start of operation, the fixed resistance r and the fixed resistance r are connected to the bridge circuit 10. It will be in a state of being. Assuming that the steady temperature in this case is T1, immediately after the relay contact 11a is opened, as shown in FIGS. 2B to 2D, the temperature of the platinum sensor 1, the current flowing therethrough, and the voltage across the platinum sensor 1 are both determined. Also approaches the steady state exponentially through the same curve. Therefore, as shown in FIG. 3 (e), sampling is performed every predetermined period to determine whether the steady state is reached because the difference between the input value and the input value immediately before is small.

If the difference becomes less than or equal to the predetermined value ε before and after the time t ₂ in step 34, the process proceeds to step 35 to switch the relay 11 to close the relay contact 11a.
Then, the fixed resistance r is not connected to the bridge circuit 10 and only the fixed resistance R is connected. At this time, the routine proceeds to step 36, where the stored heat quantity Q at the first set temperature T1 shown in the equation (1) is calculated by the outputs I ₁ , V ₁ from the A / D converters 12, 13 just before switching the relay.
Calculate 1. Q1 = (a + bU ^1/2 ) (T1-Ta) (1) = V ₁ · I ₁

[0012] The steady state temperature of the bridge circuit 10 at time t ₂ after excluding the fixed resistor r as shown in FIG. 3 (b) T2
Gradually approaches. Steps in the arithmetic processing circuit 14
37-39, as in steps 32-34, a pair of A / D
The data of the converters 12 and 13 is input and stored, and the difference between consecutive inputs is calculated. Then, it is checked whether the difference has reached a predetermined value ε, and if it exceeds this value, step 37
Return to and repeat the same processing. In this way, it is detected that the steady state has been reached immediately before time t ₃ , and the process proceeds to step 40 to switch the relay 11. In this way, the relay contact 11a is opened again and the fixed resistance r is connected in series with the fixed resistance R. Then, in step 41, the last stored input value I of the A / D converters 12 and 13
₂ and V ₂ , the amount of heat dissipated Q2 of the platinum sensor 1 at the steady temperature T2 is calculated by the following equation (2). Q2 = (a + bU ^1/2 ) (T2-Ta) (2) = V ₂ · I ₂

Then, the routine 42 is proceeded to, where the wind speed U is calculated from these radiated heat quantities Q1, Q2. The following equation is obtained by subtracting the equation (2) from the equation (1). Q1-Q2 = (a + bU ^1/2 ) (T1-T2) (3) Since T1 and T2 are known and a and b are known constants, the flow velocity U can be changed by modifying this equation. Can be measured. Then, in step 43, the flow velocity U is displayed on the display unit 15. Then, returning to step 31, the input value is cleared and the same processing is repeated. Here, the arithmetic processing circuit 14 achieves the function of the steady state determination means 16 which switches the relay by calculating the difference between the input values in steps 32 to 35 and 37 to 40 to determine whether or not the steady state has been reached. The function of the radiated heat amount calculating means 17 for calculating the radiated heat amount when the steady state is reached in steps 36 and 41 is achieved. In addition, the routine 42 uses these heat dissipation amounts Q1, Q2.
The function of the flow velocity calculating means 18 for calculating the flow velocity based on the above is achieved.

In this embodiment, the amount of heat dissipated is calculated assuming that the steady state is reached when the voltage change across the fixed resistor R1 of the bridge circuit 10 becomes small, but the current value of the platinum sensor 1 is steady. You may detect when the state is reached. Further, since the time until the steady state is reached is estimated in advance, the relay may be automatically switched by its output using an oscillation circuit having a period longer than this period. In this case, it is possible to detect the amount of heat dissipated immediately before switching and measure the flow velocity based on the difference.

In this embodiment, the sensor is a platinum sensor, but it goes without saying that another temperature sensor can be used. Also, by properly selecting the resistance value of this platinum sensor, the switching time of the bridge circuit can be made relatively short, for example, about 0.1 seconds, and it is possible to sufficiently cope with small changes in the flow velocity with time. is there.

[0016]

As described above in detail, according to the present invention, the flow velocity can be measured without the influence of the temperature of the fluid by periodically switching the steady temperature using one sensor. I have to. Therefore, according to the present invention, the spatial resolution is excellent, and since one sensor is sufficient, the adjustment and the calibration work can be easily performed.

[Brief description of drawings]

FIG. 1 is a block diagram showing an overall configuration of a hot wire anemometer according to an embodiment of the present invention.

FIG. 2 is a flowchart showing the operation of this embodiment.

FIG. 3 is a time chart showing the operation of this embodiment.

FIG. 4 is a block diagram showing a basic configuration of a conventional constant temperature hot-wire anemometer.

FIG. 5 is a block diagram showing the basic configuration of a hot-wire anemometer having a conventional temperature compensation mechanism.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Platinum sensor 2,10 Bridge circuit 3 Amplifier 11 Relay 12,13 A / D converter 14 Arithmetic processing circuit 15 Indicator 16 Steady state discrimination means 17 Dissipated heat amount calculation means 18 Flow velocity calculation means

Claims (2)

[Claims] 1. A bridge circuit having a temperature sensor having a resistance value corresponding to temperature connected at one end, switch means for changing the resistance of one side of the bridge circuit to first and second resistance values, A radiated heat amount calculating means for calculating the radiated heat amount of the temperature sensor when the connection of the bridge circuit reaches a steady value, a switch control means for automatically opening and closing the switch means, and a switch circuit connected to the bridge circuit. Flow velocity calculating means for measuring the flow velocity based on the difference in the amount of radiated heat obtained by the amount of radiated heat obtained at the first and second resistance values, respectively. 2. The switch control means is a steady state determination means for determining a steady state and opening / closing the switch means when the amount of change in the signal obtained from the bridge circuit becomes a predetermined value or less. The hot wire anemometer according to claim 1.